The present invention relates to a non-sealable, biaxially oriented multi-layer film of propylene polymers, which comprises a base layer and one or two covering layers. The invention also relates to a process for the production of this film and to its use.
The hitherto known non-sealable, biaxially oriented multi-layer films of propylene polymers, formed of a base layer and one or two covering layers (i.e. the base layer which substantially comprises propylene polymers is provided with covering layers on one or both surfaces thereof, which also substantially comprise propylene polymers) have, in particular, the disadvantage that they are unsatisfactory in view of their slip and anti-blocking characteristics, while they simultaneously have good antistatic properties and are highly transparent. In addition, the scratch resistance of these films, which is another important property, is more or less insufficient.
Many attempts have already been made to obtain polypropylene multi-layer films with improved properties by incorporating additives into the covering layers, with a view to producing the desired properties. For example, it has previously been suggested to include an effective amount of a thermoplastic resin having a melting point above the melting point of the polypropylene (incompatible thermoplastic resin) or of an inorganic pigment into the covering layers. The inorganic pigments mentioned comprise aluminum oxide, silicon dioxide, titanium dioxide, a synthetic zeolite and the like, and also micro glass beads. However, all these films are unable to meet the actual requirements.
In a recent publication (European Patent Application No. 0 124 310), a non-sealable, biaxially oriented multi-layer film of propylene polymers is described, which is stated to have good slip and antiblocking properties and, at the same time, also a high transparency. This film comprises a base layer and at least one covering layer, the covering layers containing, as an additive for the improvement of properties, a finely divided inorganic pigment, for example, silicon dioxide, aluminum silicate, sodium-aluminum silicate and/or carbon black, in an amount such that the biaxially drawn multi-layer film does no longer show any turbidity worth mentioning. The average particle size of the pigment is specified as ranging from 0.2 to 5 .mu.m. The shape (form) of the pigment particles is considered inessential.
The above-described polypropylene multi-layer film lacks, in particular, sufficient antistatic properties and scratch resistance -- not to mention its slip and anti-blocking values which still leave much to be desired. Just as all the other presently known non-sealable, oriented multi-layer films of propylene polymers, this film cannot be subjected to a corona treatment on both surfaces, since by this treatment the antiblocking behavior would be reduced to such an extent that blocking (sticking) of the film on the film roll would occur. Frequently, such sticking of superimposed film surfaces does not only give rise to elongation and tearing of the film, but can even prevent unwinding from the roll.
Moreover, sealable, biaxially oriented multilayer films of propylene polymers formed of two or three layers have also been disclosed, which contain one or several additives in their sealing layers to obtain good slip properties and, in particular, good machine runability in high-speed packaging machines, which is the most important property. These films, which are not the subject matter of the present invention, constitute a specific group of films and are intended for purposes which are completely different from those, for which nonsealable, oriented multi-layer films are used.